PROGRESS SHOWN IN EVOLUTION
ancestor of a new group or type, such being the privilege of biologically balanced or generalized creatures.
The most important progressive steps in the evolutionary ascent of animal life perhaps deserve mention. Starting from the single-celled type, life made its first great advance through the aggregation of many single cells into a colony; this advance was followed by division of labour for different functions among different kinds of cells, which gave new possibilities of size and balanced specialization of function. Next came the organization of the community of cells into a two-layered creature with a mouth at one end, a stage preserved to-day in sea-anemones and their relatives. Then came the intercalation of a third layer, and the development of a centralized (though primitive) nervous system and primitive kidneys. Then the development of a blood system, a posterior opening to the digestive tube, better locomotor organs, and elaborated sense organs in a region which might properly be called a head. Leaving all but the vertebrates out of consideration for lack of space, we would next come to the enlargement of the brain, the development of a strong internal skeleton, and then to that of paired limbs. These improvements are followed by partial emancipation from the water, as in the amphibians, then total emancipation, as in the reptiles. Still later we find the attainment of the condition of constant temperature, called warm-bloodedness; the improvement of the nourishment and care of the young, both before and after birth; and the rapid improvement of memory, associative power, and animal intelligence. Finally, in man, comes the new step in brain power which we call reason—the power of generalizing, and consequently of giving names to things, and so speech, which has brought in its train the other enormously important progressive development, the possession by the human species
[ 333 ]
